feat(downloads): ASCII-slug names, universal browser playability, force re-download

Mini-epic "Downloads correctness & UX" (S1–S3), on top of the iOS-AV1 + 720p fixes.

S1 — naming & titles:
- ASCII-fold on-disk dir/file names AND the served (Content-Disposition) filename: no
  spaces (underscore-joined), no accents (á→a, ő→o, ß→ss), pure ASCII — portable and free
  of the `?` mojibake accented names show in non-UTF-8 tools. (Forward-only; existing files
  clean up via re-download.)
- Strip a leading social engagement prefix ("1.6M views · 66K reactions | …") from titles.

S2 — universal browser playability (generalizes the iOS re-encode):
- ensure_browser_playable guarantees the stored file is H.264 + AAC/MP3 in mp4 — the only
  combo every browser decodes. Re-encode just the offending stream(s) (video for AV1/VP9/HEVC,
  audio for Opus/etc), or remux when only the container is wrong. Gated on the mp4 compat
  profile (an explicit vcodec / non-mp4 custom profile opts out).

S3 — force re-download (preserves share links):
- POST /downloads/{job_id}/redownload keeps the job row so its public DownloadLink survives
  and resolves to the freshly-downloaded file; deletes the old file and re-fetches under the
  current rules. A failed re-download leaves the job in error (the intended broken-link case).
- Library "Re-download" action (icon + confirm), api method, hu/en strings.

Tests: naming/title-strip (test_naming), browser_transcode_flags matrix; existing
download_filename expectation updated for the underscore policy.

frontend/src/components/DownloadCenter.tsx

@@ -9,6 +9,7 @@ import {
   Pause,
   Play,
   Plus,
+  RotateCcw,
   Scissors,
   Settings2,
   Share2,
@@ -672,14 +673,22 @@ export default function DownloadCenter() {
 
   const invalidate = () => invalidateDownloads(qc);
   const act = useMutation({
-    mutationFn: ({ id, op }: { id: number; op: "pause" | "resume" | "cancel" | "delete" }) =>
+    mutationFn: ({
+      id,
+      op,
+    }: {
+      id: number;
+      op: "pause" | "resume" | "cancel" | "delete" | "redownload";
+    }) =>
       op === "pause"
         ? api.pauseDownload(id)
         : op === "resume"
           ? api.resumeDownload(id)
           : op === "cancel"
             ? api.cancelDownload(id)
-            : api.deleteDownload(id),
+            : op === "redownload"
+              ? api.redownloadDownload(id)
+              : api.deleteDownload(id),
     onSuccess: invalidate,
     // No onError: a failed job action returns 400/409/422/500, already surfaced by the global
     // error modal (api.ts req) with the backend's specific message — a caller toast would double it.
@@ -824,6 +833,18 @@ export default function DownloadCenter() {
                   <IconBtn onClick={() => setShareJob(job)} title={t("downloads.actions.share")}>
                     <Share2 className="w-4 h-4" />
                   </IconBtn>
+                  <IconBtn
+                    onClick={() =>
+                      confirmThen(
+                        t("downloads.confirm.redownloadTitle"),
+                        t("downloads.confirm.redownloadBody"),
+                        () => act.mutate({ id: job.id, op: "redownload" }),
+                      )
+                    }
+                    title={t("downloads.actions.redownload")}
+                  >
+                    <RotateCcw className="w-4 h-4" />
+                  </IconBtn>
                   <IconBtn
                     onClick={() =>
                       confirmThen(